package webutility

import (
	"errors"
	"time"
	"crypto/sha256"
	"crypto/rand"
	"encoding/hex"
	"strings"
	"github.com/dgrijalva/jwt-go"
)

const OneDay   = time.Hour*24
const OneWeek  = OneDay*7
const saltSize = 32
const appName  = "korisnicki-centar"
const secret   = "korisnicki-centar-api"

// TokenClaims are JWT token claims.
type TokenClaims struct {
	Username string `json:"username"`
	Role     string `json:"role"`
	jwt.StandardClaims
}

// CredentialsStruct is an instace of username/password values. 
type CredentialsStruct struct {
	Username string `json:"username"`
	Password string `json:"password"`
}

// generateSalt returns a string of random characters of 'saltSize' length.
func generateSalt() (salt string, err error) {
	rawsalt := make([]byte, saltSize)

	_, err = rand.Read(rawsalt)
	if err != nil {
		return "", err
	}

	salt = hex.EncodeToString(rawsalt)
	return salt, nil
}

// HashString hashes input string with SHA256 algorithm.
// If the presalt parameter is not provided HashString will generate new salt string.
// Returns hash and salt string or an error if it fails.
func HashString(str string, presalt string) (hash, salt string, err error) {
	// chech if message is presalted
	if presalt == "" {
		salt, err = generateSalt()
		if err != nil {
			return "", "", err
		}
	} else {
		salt = presalt
	}

	// convert strings to raw byte slices
	rawstr := []byte(str)
	rawsalt, err := hex.DecodeString(salt)
	if err != nil {
		return "", "", err
	}

	rawdata := make([]byte, len(rawstr) + len(rawsalt))
	rawdata = append(rawdata, rawstr...)
	rawdata = append(rawdata, rawsalt...)

	// hash message + salt
	hasher := sha256.New()
	hasher.Write(rawdata)
	rawhash := hasher.Sum(nil)

	hash = hex.EncodeToString(rawhash)
	return hash, salt, nil
}

// CreateAPIToken returns JWT token with encoded username, role, expiration date and issuer claims.
// It returns an error if it fails.
func CreateAPIToken(username, role string) (string, error) {
	var apiToken string
	var err error

	if err != nil {
		return "", err
	}

	claims := TokenClaims{
		username,
		role,
		jwt.StandardClaims{
			ExpiresAt: (time.Now().Add(OneWeek)).Unix(),
			Issuer:    appName,
		},
	}

	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	apiToken, err = jwtToken.SignedString([]byte(secret))
	if err != nil {
		return "", err
	}
	return apiToken, nil
}

// RefreshAPIToken prolongs JWT token's expiration date for one week.
// It returns new JWT token or an error if it fails.
func RefreshAPIToken(tokenString string) (string, error) {
	var newToken string
	tokenString = strings.TrimPrefix(tokenString, "Bearer ")
	token, err := jwt.ParseWithClaims(tokenString, &TokenClaims{}, secretFunc)
	if err != nil {
		return "", err
	}

	// type assertion
	claims, ok := token.Claims.(*TokenClaims)
	if !ok || !token.Valid {
		return "", errors.New("token is not valid")
	}

	claims.ExpiresAt = (time.Now().Add(OneWeek)).Unix()
	jwtToken := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)

	newToken, err = jwtToken.SignedString([]byte(secret))
	if err != nil {
		return "", err
	}

	return newToken, nil
}

// ParseAPIToken parses JWT token claims.
// It returns a pointer to TokenClaims struct or an error if it fails.
func ParseAPIToken(tokenString string) (*TokenClaims, error) {
	if ok := strings.HasPrefix(tokenString, "Bearer "); ok {
		tokenString = strings.TrimPrefix(tokenString, "Bearer ")
	} else {
		return &TokenClaims{}, errors.New("Authorization header is incomplete")
	}

	token, err := jwt.ParseWithClaims(tokenString, &TokenClaims{}, secretFunc)
	if err != nil {
		return &TokenClaims{}, err
	}

	// type assertion
	claims, ok := token.Claims.(*TokenClaims)
	if !ok || !token.Valid {
		return &TokenClaims{}, errors.New("token is not valid")
	}
	return claims, nil
}

// secretFunc returns byte slice of API secret keyword.
func secretFunc(token *jwt.Token) (interface{}, error) {
	return []byte(secret), nil
}